In the industries, the conjugated diene monomers are homo-polymerized or copolymerized to form the polymers with unsaturated alkenyl groups in their molecular chains. These polymers can be beneficially used for vulcanization. However, the existences of a large amount of these unsaturated double bonds result in the poor performances of weathering resistance, heat resistance and anti-oxidation. These conditions become severe for those polymers, such as thermoplastic rubbers with physical crosslinkages, formed by polymerizing the conjugated diene monomers and vinyl aromatic hydrocarbon monomers, and more severe when used with the modifiers of styrene resins and alkenyl resins and with the impact resistance materials. These copolymers find limited applications as outdoor materials because of their poor performances of weathering resistance, heat resistance and anti-oxidation.
For the polymers formed by polymerizing the conjugated diene monomers and vinyl aromatic hydrocarbon monomers, the drawbacks of poor performances of weathering resistance, heat resistance and anti-oxidation for these polymers can be improved by the hydrogenation method to largely reduce the unsaturated double bonds. There are numerous catalysts for hydrogenating the polymers with unsaturated double bonds. These hydrogenation catalysts can be classified into two categories:
(1) heterogeneous catalysts, which are often deposited on a supporter, such as active carbon, silica, alumina, or calcium carbonate. The metal-contained heterogeneous catalysts are usually the compounds made by nickel, platinum, or palladium.
(2) homogenous catalysts, for example, (a) Ziegler-Natta catalysts composed of an organic salt of nickel, cobalt, iron, or chromium, and a reducing agent such as an organic aluminum compound, and (b) organometallic compounds containing one of Ru, Rh, Ti and La.
Although widely used in industry, heterogeneous catalysts have lower activity than the homogeneous catalyst. Therefore, in order to obtain the desired hydrogenation, a large amount of heterogeneous catalyst is required and the hydrogenation must be conducted at higher temperature and pressure. In contrast, the homogeneous catalysts are usually more active, a smaller amount of homogeneous catalysts is enough, and the hydrogenation can be conducted at mild temperature and pressure.
The hydrogenation method by using a heterogeneous catalyst is described as follows. First, the polymers to be hydrogenated are dissolved in a suitable solvent. Then, the polymers are brought into contact with hydrogen in the presence of a heterogeneous catalyst. When the polymers are hydrogenated, the viscosity of the polymers is quite high, and the contact between the polymers and the catalyst is difficult due to the interferences of the approach to the activation center by the steric hindrance of the polymers and the high absorption of the polymers, since the hydrogenated polymers tend to stay on the surface of the catalyst. Meanwhile, the hydrogenation requires higher temperature and pressure as the reactivity of the catalyst suddenly drops during the hydrogenation of the polymers, and therefore the decomposition of the polymers or the gelation of the reaction system tends to occur under such high temperature. Thus, under such operation conditions of the hydrogenation of the copolymers formed from the vinyl aromatic hydrocarbon monomers and the conjugated diene monomers, it is very difficult to selectively hydrogenate the double bonds in the conjugated diene portions. The reason is that the benzene rings in the vinyl aromatic units have been hydrogenated under such high temperature and high pressure. In addition, since the polymer is strongly adsorbed on the heterogeneous catalyst, it is impossible to completely remove the catalyst from the hydrogenated polymer solution.
In the Ziegler-Natta catalyst system, the hydrogenation reaction proceeds in the homogeneous mediums, so the hydrogenation reaction can be controlled under appropriate pressure and temperature. Thus, the hydrogenation reaction conditions can be selected to allow only the double bonds on the diene units to be hydrogenated without hydrogenating the aromatic rings on the vinyl aromatic units. However, the larger amount of the catalyst is still required. That is, the higher concentration of the catalyst is necessary. The catalyst can not be removed from the products easily, and the process of the catalyst removal is required for the post-treatment after the hydrogenation reaction, thereby making the product unstable and consuming the energy for removing the catalyst.
The conventional processes for the hydrogenation of conjugated diene polymers by using the homogeneous catalysts are summarized below.
U.S. Pat. No. 4,980,421 discloses a process for selectively hydrogenating a conjugated diene polymer by using the hydrogenation catalyst including a bis(cyclopentadienyl) titanium(+4) compound, an alkoxy lithium compound, and an organometallic compound (such as aluminum, zinc, or magnesium compound). This hydrogenation catalyst has high activity, and thus a small amount is required to reach the hydrogenation effect. Moreover, the deash process is not required and the reaction can proceed under mild conditions for this hydrogenation catalyst.
U.S. Pat. No. 5,270,274 discloses a hydrogenation catalyst composition including a bis(cyclopentadienyl) titanium(+4) compound, a compound containing polar carbonyl groups and epoxy groups, and an organic lithium compound. The unsaturated double bonds in the conjugated diene polymer can be preferentially hydrogenated. The hydrogenated polymers have superior physical properties and weathering resistance.
U.S. Pat. No. 5,244,980 discloses a hydrogenation process including terminating a living conjugated diene polymer with hydrogen and then adding an organo alkali metal and a Tebbe's catalyst, which has good hydrogenation efficiency is disclosed therein.
U.S. Pat. No. 5,886,108 discloses hydrogenating a living conjugated diene polymer introduced with hydroxyl, carbonyl or ester group by using the Tebbe's catalyst prepared by the reaction of a bis(cyclopentadienyl) titanium(+4) compound and trimethyl aluminum. This patent discloses that the hydrogenation efficiency is good when the mentioned compound is included in the reaction.
U.S. Pat. No. 5,985,995 discloses a catalyst composition. When a living conjugated diene polymer is combined with bis(cyclopentadienyl)Ti(PhOCH3)2 and alkyl silicon halide or alkyl tin halide. This patent discloses that the reaction reacted with the mentioned catalyst composition has very good hydrogenation efficiency.
U.S. Pat. No. 5,948,869 discloses a selective hydrogenation of unsaturated double bonds in conjugated diene polymers, where the catalyst composition includes a bis(cyclopentadienyl) titanium(+4) compound, an alkyl zinc or alkyl magnesium, and the promoter of the solvent with ether or aromatic groups. This patent discloses that the mentioned catalyst composition increases the hydrogenation efficiency obviously.
European Patent Appl. No. 0434469A2 discloses a catalyst composition for hydrogenating a conjugated diene polymer, where the catalyst composition includes a bis(cyclopentadienyl) titanium compound, an alkoxy alkali metal, and a polar compound of ether, ketone or other types. The catalyst system has capacities of hydrogenating conjugated diene polymers and the copolymers thereof.
European Patent Appl. No. 0544304A1 discloses a catalyst composition including (a) a bis(cyclopentadienyl) transition metal compound; (b) a polar compound containing carbonyl groups or epoxy groups such as esters of monoacids or diacids, internal ester, internal amide or epoxy; (c) an organic lithium compound; and (d) a reducing organometalic compound such as aluminum compound, zinc compound, or magnesium compound, for example, triethyl aluminum. This patent discloses that the mentioned catalyst composition increases the hydrogenation efficiency obviously.
U.S. Pat. No. 6,313,230 discloses the selective hydrogenation of the unsaturated double bonds of conjugated diene polymers, where the catalyst composition includes a bis(cyclopentadienyl) titanium(+4) compound and a tri-substituted silane with Si—H functional group. It is described that this catalyst composition can promote the hydrogenation efficiency. However, the silane contains hydrogen, thus tends to be hydrolyzed easily and reacted with oxygen or polar functional groups easily, and accordingly is hard to be stored at room temperature. In addition, from the embodiments of this patent, it is known that the highest reaction temperature is 60° C., the hydrogenation reaction cannot continuously proceed under high temperature, and the catalyst composition therein would deactivated under high temperature hydrogenation reaction. Thus a large amount of heat during the hydrogenation reaction must be removed; otherwise the hydrogenation efficiency will be low. This catalyst composition can not be stored under nitrogen gas for a long time, and the activity of the catalyst easily decays. The catalyst composition after the preparation must be used in short time; otherwise the hydrogenation rate can not be easily controlled. According, this catalyst composition is not suitable for the continuous manufacture.
According to the above descriptions, although many catalyst compositions had been used for hydrogenating a polymer formed by a conjugated diene, several problems of instability of the products and tedious procedures to remove the catalyst due to the large amount of the catalyst required. In order to solve the above-mentioned problems, after a lot of experiments and analyses, the research and development team of the present invention has developed novel hydrogenation methods with the catalyst compositions, which are stable and can be easily stored. In the present invention, only a small amount of the catalyst composition is needed, the removal of the catalyst from the hydrogenated compound is no longer required, and the polymers of conjugated dienes can be successfully hydrogenated. Moreover, the catalyst compositions of the present invention still have high catalysis activities at high temperature, and therefore are very suitable for the commercial mass production.